Denoising options in latest 2.91 nightly builds
The short answer is that if you enable the Denoising in the render options you do not need to use the compositor Denoise node. And OpenImageDenoise is better.
The longer answer is that OpenImageDenoise is the same as the compositor node but it works while rendering rather then after the the render which can speed things up a lot. NLM is a denoiser that doesn't use ai unlike the others so it isn't as good.
This is a version of a project i started that has subsurface scattering so it is harder to render
This is with only 5 samples and no denoising

This is all the same and still with 5 samples but with NLM denoiser which is the one that doesn't use ai

And this is identical but with the OpenImageDenoiser made by Intel that uses ai but can sometimes be slightly slower than Optix ai denoiser by Nvidia that is only for compatible Nvidia graphics cards and slightly less impressive
As you see the OpenImageDenoise is better
For curiosity's sake this is the faster but only for Nvidea cards Optix real-time denoiser that is somewhere in between NLM and Intel OpenImageDenoise